For pure nanotubes add water

By
Eric Smalley,
Technology Research NewsWashing
away impurities with water turns out to be as good for growing carbon nanotubes
as it is for keeping a clean house.

Carbon nanotubes show great promise as building blocks for molecular
machines, high-speed electronics and super-strong materials, but it has
proven difficult to reliably grow large amounts of pure carbon nanotubes
and to keep the growth process orderly.

The long, rolled-up sheets of carbon atoms can be narrower than
a single nanometer, have useful electrical, optical, and magnetic properties,
and are stronger by weight than steel. A nanometer is one millionth of a
millimeter, or the span of 10 hydrogen atoms.

Researchers from the Japanese National Institute of Advanced Industrial
Science and Technology (AIST) have added water to the standard method of
manufacturing carbon nanotubes to produce tall, dense, vertically-aligned
stands of pure nanotubes. The researchers' demonstrations have yielded stands
of single-wall carbon nanotubes as tall as 2.5 millimeters and 99.98 percent
pure. Individual nanotubes range from one to three nanometers in diameter.

The purity of the nanotubes makes the usual post-growth purification
process unnecessary. This makes the method quicker, less expensive and less
likely to damage the nanotubes than existing processes, said Kenji Hata,
a senior researcher at the Japan National Institute of Advanced Industrial
Science and Technology. Nanotubes produced using the method are orderly
and pure enough for use in many fields, including biology, medical implants,
chemistry, electronics and magnetics research, he said.

Carbon nanotubes are typically grown using chemical vapor deposition
techniques. These involve heating surfaces like silicon wafers or metal
foil that contain microscopic metal particles in the presence of gases like
methane or ethylene. The metal particles act as catalysts that extract carbon
atoms from the gases; the carbon atoms then naturally form nanotubes.

Ordinarily growth stops after about one minute as disorganized carbon
soot accumulates. Water removes this amorphous carbon layer, making more
catalysts particles active so that more nanotubes grow, and keeping the
catalysts active longer to produce taller nanotubes. "This synthesis is
highly efficient, meaning that almost all the catalysts on the surface are
active in growing tubes," said Hata.

The high density of the nanotubes causes them to grow vertically
rather than in many directions. "This is all due to the use of water," said
Hata. If the number of nanotubes in a given space is too low "the tubes
would not be able to stand and form these structures [and] you would end
up with a pile of spaghetti," he said.

The nanotube structures can be easily removed from the surfaces
and the catalysts used again to produce new structures, said Hata.

Because the water keeps amorphous carbon from the samples, the process
does not require a step to remove the amorphous carbon.

The researchers' demonstrations have shown that growth can continue
for as long as 30 minutes. Their fastest growth to date is a 10-minute 2.5-millimeter
sample.

The researchers used lithography techniques to pattern the catalysts
on surfaces in order to grow various three-dimensional structures, including
arrays of cylinders and sheets. The researchers produced cylinders 1 millimeter
tall and about one third of a millimeter in diameter. They also produced
10-micron thick sheets that can be laid flat to form thin films of pure
carbon.

The method could be used to mass produce carbon nanotubes within
five years, and for practical applications within ten years, said Hata.

Hata's research colleagues were Don N. Futaba, Kohei Mizuno, Tatsunori
Namai, Motoo Yumura and Sumio Iijima. The work appeared in the November
19, 2004 issue of Science. The research was funded by the Japan National
Institute for Advanced Industrial Science and Technology (AIST) and the
New Energy and Industrial Technology Development Organization (NEDO).